Apparatus for exposure of photo-curing resin applied to printed circuit board

ABSTRACT

In exposing a photocurable resin to light, the light is irradiated on the photocurable resin while a base material to which the photocurable resin is applied is immersed in a liquid. An apparatus for carrying out the exposure includes an exposure tank in which a liquid does not dissolve the photocurable resin is reserved and a light source irradiating light to the photocurable resin is immersed in the liquid reserved in the tank. The exposure tank has two opposite side walls which have exposure windows closed by transparent plates respectively. The apparatus may include a plurality of light sources disposed so as to correspond to the representative exposing window. A temperature of the liquid in which the base material is immersed is controlled.

This application is a divisional of application Ser. No. 09/005,233,filed Jan. 9, 1998, now U.S. Pat. No. 6,165,544.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to a method of exposing a photo-curing type resinto light, an apparatus therefor, and a method of fabricating a printedcircuit board by use of the exposing method.

2. Description of the prior art

In order that an ultraviolet-curing type ink may be applied to a surfaceof a printed circuit board to be hardened, the printed circuit board towhich the ink has been applied is conventionally be carried by aconveyor. The printed circuit board is exposed to ultraviolet light fromultraviolet lamps during the conveyance.

The above-described method has the following drawbacks. First, radiantheat from the ultraviolet lamps increases a temperature of the ink,resulting in occurrence of air bubbles in the resin. The air bubblessometimes reside as voids in the hardened resin.

Second, the increase in the temperature of the ink exposed to theultraviolet light promotes or hastens hardening. This causes bridgingdensity more than required and accordingly, excessively increases thehardness of the ink. When the hardness of the hardened ink isexcessively increased, difficulty accompanies a step of grinding thehardened ink.

Third, when the ink is screen-printed on the printed circuit board, forexample, an atmospheric pressure is merely applied to the ink after thescreen printing. Accordingly, minute air bubbles contained in the inkduring the screen printing cannot sufficiently be eliminated. The ink isthus hardened with the minute air bubbles contained therein.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to solve theabove-described drawbacks and more particularly, to provide a method ofexposing a photo-curing type resin wherein the temperature of thephoto-curing type resin during exposure to light can be controlled sothat the voids and the excessive bridging density resulting from thetemperature increase are prevented, and an apparatus therefor.

In one aspect, the present invention provides a method of exposing aphoto-curing type resin which is hardened when exposed to light,comprising the step of exposing the resin to light while a base materialto which the photo-curing type resin is applied is immersed in a liquid.

A liquid has a larger heat capacity and a larger specific heat than agas. Accordingly, the liquid has a heat sinking property restrainingvariations in a temperature of a body in contact therewith superior tothe gas. Since the light is irradiated to the photo-curing type resinwhile the resin is immersed in the liquid, the variations in thetemperature of the resin is restrained by the heat sinking property ofthe liquid in contact therewith. Furthermore, liquid pressure is appliedto the photo-curing type resin from around the same in the liquidreserved in the exposure tank. Consequently, even when minute airbubbles are contained in the resin as the result of the screen printing,the air bubbles can be eliminated from the resin layer by the liquidpressure applied to the resin.

In order that the above-described method may be carried out, an exposuretank reserving the liquid has a side wall which has an exposure windowclosed by a transparent plate, and the light from a light sourcedisposed outside the exposure tank is irradiated through the transparentplate to the photo-curing type resin. Since the light source isprevented from coming into contact with the liquid by an insulatingstructure, the maintenance of the light source can be simplified.

Furthermore, a liquid circulating passage is preferably provided throughwhich the liquid reserved in the exposure tank is circulated, and a heatexchanger is preferably provided in the liquid circulating passage forexchanging heat of the liquid so that a temperature of the liquid ismaintained in a predetermined range. Consequently, the bridging densityof the resin can be controlled to take a predetermined value.

Other objects, features and advantages of the present invention willbecome clear upon reviewing the following description of preferredembodiments thereof, made with reference to the accompanying drawings,in which:

FIG. 1 is a sectional view of an exposure tank employed in the exposingapparatus of a first embodiment in accordance with the presentinvention;

FIG. 2 is a view taken along line 1—1 in FIG. 2;

FIGS. 3A and 3B are enlarged sectional views showing a through holeforming step in the fabrication of a printed circuit board in accordancewith a second embodiment of the invention;

FIGS. 4A and 4B are enlarged sectional views showing a hole conductorlayer forming step in the fabrication of the printed circuit board;

FIGS. 5A, 5B and 5C are enlarged sectional views showing a circuitpattern forming step in the fabrication of the printed circuit board;and

FIGS. 6A, 6B and 6C are enlarged sectional views showing a circuitpattern embedding step and a grinding step in the fabrication of theprinted circuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will be described withreference to FIGS. 1 and 2. The embodiment is directed to thefabrication of a printed circuit board having through holes filled witha photo-curing type resin.

Referring to FIG. 1, an exposing apparatus comprising an exposure tank 1having an upper opening is shown. Water is reserved in the exposure tank1. The exposure tank 1 has a pair of opposite side walls 2 formed withrespective exposure windows 3. Transparent plates 4 are fitted in theexposure windows 3 respectively. Two light sources 5 each comprising anultraviolet lamp are disposed outside the respective transparent plates4 so that ultraviolet light is irradiated through the exposure windows 3into the exposure tank 1.

A portion of the exposure tank formed with the exposure windows 3 has areduced width as shown in FIG. 2. For example, a distance between thetwo transparent plates 4 is set at about 20 cm. A conveyor (not shown)is provided over the exposure tank 1. A printed circuit board 6 conveyedon the conveyor is immersed in the water in the right-hand interior ofthe exposure tank 1. The printed circuit board 6 is then conveyedthrough a water channel between the exposure windows 3 to the left. Theprinted circuit board 6 is lifted up at the left-hand interior of theexposure tank 1.

A circulation pipe passage 9 communicates with the exposure tank 1. Thecirculation pipe passage 9 has a circulation pump 7 and a heat exchanger8. The circulation pump 8 is operated to circulate the water in theexposure tank 1 through the heat exchanger 8. The heat exchanger 8exchanges heat between the water in the exposure tank 1 and refrigerantcooled by an evaporator (not shown). As the result of the heat exchange,a temperature of the water in the exposure tank 1 is maintained in apredetermined range, for example, a range between 10 and 20° C.

The method of exposure by use of the above-described exposing apparatuswill now be described. Processes for printing an ultraviolet-curing typepermanent resist ink on the printed circuit board 6 and hardening theprinted ink will be described. The permanent resist ink used in theembodiment is of an epoxy indissoluble type and is printed by a normalscreen printing. When the printed circuit board 6 has through holes, thepermanent resist ink is applied to the printed circuit board 6 so as tofill the through holes.

The printed circuit board 6 to which the permanent resist ink has beenapplied is conveyed over the exposure tank 1 by the conveyor. First, theprinted circuit board 6 is immersed in the water in the right-handinterior of the exposure tank 1. Since the permanent resist ink isindissoluble in water, the immersion does not result in any chemicalchanges in the ink. However, a temperature of the permanent resist inkis approximately equalized with the water temperature. Furthermore,water pressure is uniformly applied to the ink when the printed circuitboard 6 is immersed in the water. Consequently, minute air bubblesunavoidably contained in the permanent resist ink during the screenprinting may be forced out by the water pressure.

The printed circuit board 6 is then moved in the water between theexposure windows 3 in the exposure tank 1 During the movement of theprinted circuit board 6 through the water channel between the exposurewindows, the ultraviolet light from the light sources 5 is irradiated toa resist layer so that hardening is started. A time period of theultraviolet irradiation is equal to a time period required for securingan amount of ultraviolet light necessary for the hardening of the resistink. The ultraviolet light is generally irradiated for a short timeperiod from one minute to one minute and thirty seconds. A large radiantheat energy from the light sources 5 tends to increase the temperatureof the resist layer during the ultraviolet irradiation. In theembodiment, however, the resist layer is immersed in the low-temperaturewater. Furthermore, a wide area of the resist layer is in contact withthe water. Consequently, the increase in the temperature of the resistink layer is reliably restrained by the heat sinking property of thewater. The temperature of the resist ink layer is within the set rangeof the water temperature between 10 and 20° C.

Thus, since substantially no air bubbles are formed in the ink layer dueto a rapid increase in the temperature, the voids can reliably beprevented from remaining in the hardened resin layer. This sufficientlyimproves the characteristics of the permanent resist of the printedcircuit board, for example, the heat-resistance, moisture-resistance,electrical resistance, etc. As a result, a high quality of resist layercan be obtained.

Furthermore, since the ink is hardened under the condition at thepredetermined temperature, the bridging density thereof can be within anexpected range. Consequently, the permanent resist layer can beprevented from being excessively hardened by an excessive increase inthe bridging density. Accordingly, the grinding step can readily becarried out without difficulty after the hardening step. Furthermore,since the light sources 5 are disposed outside the exposure tank 1 andaccordingly, separated from the water in the tank, an insulatingstructure of each light source 5 and accordingly the maintenance thereofcan be simplified. Additionally, since an amount of heat generated byeach light source 5 to penetrate into the exposure tank 1 is reduced,the temperature of the water can readily be controlled. This increasesthe accuracy in the control of the temperature of the permanent resistlayer during the hardening.

Upon completion of the exposure, the printed circuit board 6 is liftedout of the exposure tank 1 by the conveyor and moved to a drier (notshown) for drying.

Although the water is reserved in the exposure tank 1 in the foregoingembodiment, another liquid which does not dissolve the photo-curing typeresin may be used, instead. For example, when the light irradiationunder the condition at a low temperature is preferred, liquefiednitrogen may be used. Furthermore, liquids of the alcohol system such asmethanol, ethanol, or isopropyl alcohol may be used depending upon thetypes of the photo-curing type resins. Other usable liquids includeliquids of the hydrocarbon system such as heptane, mineral spirit,liquid paraffin, and xylene, liquids of the halogen system such asmethylene chloride, trichloroethane, perchloroethylene, methyl bromide,propyl bromide, Freon 113, HCFC 225, and xylenehexachloride, polyvalentalcohol derivatives such as ethylene glycol and ethylene glycol dimethylether, and oils such as terepyne oil, kerosene, and silicon oil. Amixture of these liquids may be used. Additionally, various additivessuch as antiseptic may be added to these liquids.

Although the epoxy indissoluble type permanent resist ink is exposed tothe light in the foregoing embodiment, the invention may be applied to aphenolic ultraviolet-curing type ink. Furthermore, the invention may beapplied to various resins which are exposed to light to be hardened.

The temperature of the water in the exposure tank 1 ranges between 10and 20° C. in the foregoing embodiment. However, the water temperaturemay be increased as the hardening demands and set at a desired value.Furthermore, a liquid with a high specific gravity may be selected sothat the pressure applied to the photo-curing type resin in the liquidis increased. In this case, the air bubble elimination performance canfurther be improved.

FIGS. 3A to 6C illustrate a second embodiment which is directed to thefabrication of a printed circuit board by a subtractive process. Acopper-clad laminate is used as the printed circuit board 6 in thesecond embodiment. The copper-clad laminate comprises the base materialsuch as a glass epoxy substrate 11 and copper foils 12 cladded to bothsides of the substrate and serving as conductive layers respectively, asshown in FIG. 3A.

The following steps are sequentially executed for the printed circuitboard 6. Through hole forming step:

Through holes 13 are formed in the printed circuit board 6 by means of aknown drill (not shown). Plating step:

The printed circuit board 6 is then immersed in a chemical platingliquid so that a chemically plated layer 14 is formed over an entirearea thereof including inner peripheral faces of the through holes 13,as shown in FIG. 4A. Since the chemically plated layer 14 is connectedto the copper foils 12, an electroplated layer 15 of copper is formed onthe chemically plated layer serving as a base electrode, as shown inFIG. 4B. Consequently, hole conductor layers 16 are formed on the innerperipheral faces of the through holes 13 respectively. Circuit patternforming step:

Subsequently, an unnecessary portion of the conductive layer is removedso that a circuit pattern is formed. A known photoetching process can beemployed for this purpose. More specifically, a photo-sensitive etchingresist 17 is first printed on the printed circuit board 16. After havingbeen dried, the resist 17 is exposed to light with a circuit patternfilm being laid thereon. Upon development, a wiring substrate isobtained as shown in FIG. 5A. In the obtained wiring substrate, thehardened etching resist 17 is laid on a portion of the substrate to beleft as the circuit pattern.

The printed circuit board 6 is then immersed in an etching liquid sothat the electroplated layer 15, the chemically plated layer 14 and thecopper foils 12 are dissolved to be removed, as shown in FIG. 5B. Whenthe etching resist 17 is then removed, desired circuit patterns 18 areformed, as shown in FIG. 5C. Conductor embedding step:

A resin material is then applied to the surface of the printed circuitboard 6 so that the printed circuit patterns 18 are embedded in theresin material. The epoxy indissoluble resist ink of theultraviolet-curing type type is used as the resin material. This resistink can be applied to the surface of the printed circuit board 6 by thenormal screen printing process.

A resist ink 19 is first printed on one side of the printed circuitboard 6 so that the circuit patterns 18 are embedded in the resist ink19 as shown in FIG. 6A. In this case, the through holes 13 of theprinted circuit board 6 are also filled with the resist ink 19. Exposingstep:

The resist ink 19 is then exposed to light by the exposing apparatussuch as employed in the first embodiment to thereby be hardened. Theresist ink 19 is also printed on the opposite side of the printedcircuit board 6 so that the circuit patterns 18 are embedded in theresist ink 19 at both sides of the printed circuit board 6. The resistink 19 is again exposed to light to be hardened, as shown in FIG. 6B.

The printed circuit board 6 to which the resist ink 19 has been appliedis conveyed over the exposure tank 1 as shown in FIG. 1 by the conveyor.First, the printed circuit board 6 is immersed in the water in theright-hand interior of the exposure tank 1. Since the resist ink 19 isindissoluble in water, the immersion does not result in any chemicalchanges in the ink 19. However, a temperature of the resist ink 19 isapproximately equalized with the water temperature. Furthermore, waterpressure is uniformly applied to the ink 19 when the printed circuitboard 6 is impregnated in the water. Consequently, minute air bubblesunavoidably contained in the resist ink 19 during the screen printingmay be forced out by the water pressure to thereby be moved near thesurface.

The printed circuit board 6 is then moved in the water between theexposure windows 3 in the exposure tank 1. During the movement of theprinted circuit board 6 through the water channel between the exposurewindows, the ultraviolet light from the light sources 5 is irradiated tothe resist ink 19 so that hardening is started. A time period of theultraviolet irradiation is equal to a time period required for securingan amount of ultraviolet light necessary for the hardening of the resistink 19. The ultraviolet light is generally irradiated for a short timeperiod from one minute to one minute and thirty seconds. A large radiantheat energy from the light sources 5 tends to increase the temperatureof the resist layer during the ultraviolet irradiation. In theembodiment, however, the resist ink 19 is immersed in thelow-temperature water. Furthermore, a wide area of the resist ink 19 isin contact with the water. Consequently, the increase in the temperatureof the resist ink 19 is reliably restrained by the heat sinking propertyof the water. The temperature of the resist ink 19 is within the setrange of the water temperature between 10 and 20° C.

Thus, substantially no air bubbles are formed in the resist ink 19 dueto a rapid increase in the temperature. This, together with the movementof the air bubbles near the surface as described above, can reliablyprevent the voids from remaining in the hardened resin layer. Thissufficiently improves the characteristics of the hardened resist of theprinted circuit board, for example, the heat-resistance,moisture-resistance, electrical resistance, etc. As a result, a highquality of resist 19 can be obtained.

Furthermore, since the ink 19 is hardened under the condition at thepredetermined temperature, the bridging density thereof can be within anexpected range. Consequently, the resist 19 can be prevented from beingexcessively hardened by an excessive increase in the bridging density.Furthermore, since the light sources 24 are disposed outside theexposure tank 20 and accordingly, separated from the water in the tank,an insulating structure of each light source 24 and accordingly themaintenance thereof can be simplified. Additionally, since an amount ofheat generated by each light source 24 to penetrate into the exposuretank 1 is reduced, the temperature of the water can readily becontrolled. This increases the accuracy in the control of thetemperature of the resist 19 during the hardening.

Upon completion of the exposure, the printed circuit board 6 is liftedout of the exposure tank 20 by the conveyor and moved to a drier (notshown) for drying. Grinding step:

The grinding step is finally carried out after the resist 19 has beenhardened. A grinding machine such as a belt thunder or buff grinder isused. The resist 19 hardened so as to cover the circuit patterns 18 issmoothly ground until the circuit pattern 18 is exposed on the surface.Although the upper surface of the circuit pattern 18 is exposed afterthe grinding, regions between the circuit patterns 18 are buried by theresist 19, as shown in FIG. 6C.

Since the air bubbles gather near the surface of the hardened resist 19as described above, the resist 19 is desirably ground even when it ishardened. Furthermore, the electric characteristics of the resist 19 canbe maintained at a sufficiently high level since the number of airbubbles remaining or produced therein is small.

Subsequently, the fabrication of the printed circuit board 6 iscompleted after later steps such as washing and drying.

According to the second embodiment, the regions between the circuitpatterns 18 are buried by the resist 19, and the circuit patterns andthe hardened resist 19 are at the same level such that the surface ofthe printed circuit board 6 is rendered flat. In other words, since thesides of the conductors constituting the circuit patterns 18 are coveredwith the resist 19, penetration of solder to the sides of the circuitboard can be prevented. Consequently, the method of the secondembodiment can achieve the same narrow pattern pitch as in a printedcircuit board fabricated by the full-additive process. Moreover, thecircuit patterns 18 are formed by the etching process in which theunnecessary portions of the copper foils 12 are removed as in thesubtractive process. Consequently, a higher productivity and a lowerproduction cost can be accomplished.

Since the through holes 13 are filled with the resist 19, thecharacteristics thereof can be prevented from being reduced due to entryof foreign matters thereinto. As a result, a high performance of printedcircuit board can be provided.

Although the through holes are formed in the printed circuit board 6 inthe foregoing embodiment, they may or may not be formed. Theabove-described through hole forming and plating steps can be eliminatedwhen no through holes are formed in the printed circuit board.

The foregoing description and drawings are merely illustrative of theprinciples of the present invention and are not to be construed in alimiting sense. Various changes and modifications will become apparentto those of ordinary skill in the art. All such changes andmodifications are seen to fall within the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. An apparatus for exposing a photocurable resinwhich is hardened when exposed to light, comprising: an exposure tank inwhich a liquid which does not dissolve the photocurable resin isreserved; and a light source for irradiating light to the photocurableresin to be immersed in the liquid reserved in the tank, wherein theexposure tank has a side wall which has an exposure window defined by atransparent plate, and the light from the light source is irradiatedthrough the transparent plate to the photocurable resin from outside theexposure tank.
 2. An apparatus for exposing a photocurable resin whichis hardened when exposed to light, comprising: an exposure tank in whicha liquid which does not dissolve the photocurable resin is reserved; anda light source for irradiating light to the photocurable resin to beimmersed in the liquid reserved in the tank, wherein the exposure tankhas two opposite side walls which have exposure windows defined bytransparent plates respectively, and which further comprises a pluralityof light sources disposed so as to correspond to the respective exposingwindows.
 3. The apparatus according to claim 1, further comprising aliquid circulating passage through which the liquid reserved in theexposure tank is circulated, and a heat exchanger provided in the liquidcirculating passage for exchanging heat of the liquid so that thetemperature of the liquid is maintained in a predetermined range.
 4. Theapparatus according to claim 2, further comprising a liquid circulatingpassage through which the liquid reserved in the exposure tank iscirculated, and a heat exchanger provided in the liquid circulatingpassage for exchanging heat of the liquid so that the temperature of theliquid is maintained in a predetermined range.